Most women are told a lot about how not to get pregnant, and almost nothing about how pregnancy actually happens. The result is that when you do want to conceive, you are often working from a half-built understanding of your own biology. Here is the version you should have been given years ago.
What Has to Happen for Pregnancy to Occur
Four things need to line up. An egg has to be released from the ovary. Sperm has to be present in the right place at the right time. The egg and sperm have to meet, usually in the fallopian tube and fertilisation has to occur. The resulting embryo then has to travel down to the uterus and implant successfully in the uterine lining.
That is a lot of steps, and each one is its own small biological event. Even in a healthy couple in their late twenties, the chance of pregnancy in any given month is only around 20 to 25 per cent. That number drops with age, which is the most under-discussed part of the conversation.
The Fertile Window
Here is the part many of us were taught wrong at school. You cannot get pregnant on every day of your cycle. Ovulation, the release of an egg, happens once per cycle, and the egg only survives for around 12 to 24 hours after it is released. So, the actual window in which conception is possible is short.
Sperm, on the other hand, can survive in the reproductive tract for up to five days under the right conditions. This means the fertile window is roughly six days long, spanning the five days leading up to ovulation and the day of ovulation itself. The two or three days before ovulation are when the chance of conceiving is highest.
In a 28-day cycle, ovulation happens around day 14, but most cycles are not textbook. Ovulation can vary from cycle to cycle even in the same person, which is why tracking is far more useful than relying on calendar maths alone.
Why Egg Quality Comes Up So Often
Sperm is produced continuously throughout a man’s life. Eggs work differently. You were born with all the eggs you will ever have, around one to two million at birth, dropping to around 300,000 to 400,000 by puberty. From that point, the number declines steadily, and the quality of the remaining eggs declines too, more noticeably from the mid-thirties onward.
Egg quality refers to the chromosomal health of the egg. A genetically healthy egg is more likely to fertilise, more likely to develop into a viable embryo, and more likely to implant and result in a live birth. As eggs age, the rate of chromosomal abnormalities rises, which is why the chance of miscarriage also rises with age and why some IVF cycles do not progress past the embryo stage.
This is not a reason to panic. It is a reason to be informed.
What Supports Fertility
Beyond the basics of ovulation and sperm, several things have good evidence behind them:
Not smoking, for both partners
Keeping alcohol intake low (preferably to none), particularly in the months leading up to trying
Maintaining a body composition that supports regular ovulation, since both very low and very high body fat can disrupt the cycle
Eating in a way that supports steady blood sugar, since insulin resistance interferes with ovulation
Getting enough sleep, since poor sleep raises cortisol and can suppress reproductive hormones
Folate before and during conception, ideally started three months in advance
When to Speak to Someone
If you are under 35 and have been trying for 12 months without success, or over 35 and have been trying for six months, it is worth seeing a clinician. If you already know you have irregular cycles, a history of pelvic infection, endometriosis, fibroids, or other reproductive conditions, do not wait the full year. Earlier conversations open up earlier options, and time is one of the things you cannot get back in fertility care.